Spherical aberration is an optical distortion that occurs as a result of using optical lenses with a spherical surface. Light that passes through the lens near the center is refracted at less of an angle than light passing through the lens at the edge. The result is light being focused at different points along the optical axis instead of in one location.
The image above shows a lens that suffers from spherical aberration. Instead of all of the light being focused at the optimal focus point, it is spread across various points along the axis. Spherical Aberration can blur details, soften contrast and make it difficult to produce a sharp image. The results can be seen across the entire image and not just in the center.
Ideally, a perfect lens would focus all of the light at the same point along the optical axis. When light is refracted at too great an angle it’s called positive spherical aberration. Light that isn’t refracted at a great enough angle is called negative spherical aberration.
Spherical shaped lenses are cheap and easy to manufacture, but they aren’t great at producing sharp images. The quality of glass used can also have an impact on the end result.
Lens manufacturers use several techniques to minimize spherical aberrations. Sometimes lens elements are bent or used in groups to reduce the effects. One of the better techniques uses an aspheric lens. An aspheric lens has a non-spherical surface that corrects for distortion across the entire surface of the lens. The surface of an aspheric element looks like a bell curve with a large bulge in the center of the surface. Aspheric elements can completely eliminate spherical aberration.
Reducing Spherical Aberration
If you own a lens that has problems with spherical aberration, the best thing you can do is stop the aperture down. Prime lenses, wide-angle lenses, and lenses with fast (wide) maximum apertures usually suffer the worst from spherical aberration.
Spherical aberration is worst when the lens’ aperture is wide open. Stopping down the aperture just one stop can drastically improve image quality. The aperture blades block light from the outermost part of the lens’ surface and reduce the effect to a minimum.
You might also enjoy this article on chromatic aberration.